Federal law has for some years required that motor vehicles be fitted with auxiliary mechanical brake systems to supplement the hydraulic service brakes normally employed. Typically these auxiliary or "emergency" brakes are operated by a pivoted lever mounted for gripping by the driver's hand or as a pedal for depressing with the driver's foot. Commonly such operating levers are connected to the shoes of the mechanical brakes by flexible cables. Over time these cables tend to stretch, and the brake shoes wear, which can result in an inconvenient amount of lever travel being required to fully actuate the brakes. Accordingly, numerous patents have been issued directed to self-adjusting brake cable mechanisms.
For example, U.S. Pat. No. 3,109,320 to Krautwurst shows a relatively complex mechanism for automatically taking up the slack in a brake cable. As is commonly the case, the Krautwurst device involves a pawl-and-ratchet mechanism for locking the operating lever to set the brake. That is, when the operator desires to set the brake, he pulls on a lever attached to a sheave, which pulls on the cable, setting the brake. A pawl carried by the lever clicks over the teeth of a stationary ratchet. When the lever is moved a distance sufficient to set the brake adequately, the pawl holds the lever in the actuated position. When the operator then desires to release the auxiliary brake, he must pull the lever somewhat further and press a button or otherwise move the pawl out of engagement with the ratchet, so that the lever can move back to its rest position. This requires that some slack be left in the cable even after the brake is fully actuated.
The Krautwurst system takes up the slack resulting from cable stretch or brake wear at the brake end, that is, the slack is taken up during return of the lever to its rest position. This is done by altering the position of the cable with respect to the brake actuating member. This can result in no slack being available to allow releasing of the brakes.
U.S. Pat. No. 1,980,771 to Tibbetts shows a self-adjusting rod-actuated brake for a vehicle, the teachings of which would seem applicable to cable-operated brakes. The Tibbetts mechanism also involves adjustment on the return stroke; see p. 2, lines 56-61. This system would accordingly involve the same deficiencies noted with respect to the Krautwurst disclosure.
Numerous other cable adjusting mechanisms are also known to the art, many of which are disclosed in connection with automotive clutch cables. In many cases clutch actuation and auxiliary brake actuation mechanisms are comparable. However, it will be appreciated that normally the force required to actuate a clutch is much less than that required for actuation of brakes. Accordingly, the teachings of the clutch actuation art cannot necessarily be employed in auxiliary brake applications.
Patents generally teaching clutch adjustment mechanisms involving cables include Gale et al. U.S. Pat. No. 3,621,959; Wheaton U.S. Pat. No. 4,181,209; Grunberg et al. U.S. Pat. No. 4,671,400; and Smirl et al. U.S. Pat. No. 3,365,042.
Clutch actuation mechanisms which are less pertinent are shown in U.S. Pat. No. 3,307,667 to Maurice, showing an automatic adjustment device for a hydraulic actuated clutch, and Miller U.S Pat. No. 2,616,540 showing an automatic wear compensator for a directly actuated clutch, that is in which no cable is used. Wemp U.S. Pat. No. 2,036,004 also shows a self-adjusting mechanically actuated clutch mechanism.
An auxiliary brake actuator mechanism which is currently in commercial use appears to be generally as disclosed in the Gale patent, and suffers from the difficulties noted. This mechanism comprises a hand-operated lever which is rotatable about an axis on a frame member which is fixed to the vehicle. The frame member comprises a fixed sector, that is, an arcuate ratchet section, which interacts with a first pawl pivotally mounted on the control lever. The interaction of the first pawl with the fixed sector is such that the lever can be pivoted by the driver from a rest position about the axis to wind the brake cable around a sheave to set the brake. The pawl then holds the lever. When the operator later desires to release the brake, he presses a button which releases the first pawl from the ratchet and allows the lever to pivot back to its rest position.
In this device, the sheave is mounted concentrically with the axis of the lever. The sheave carries a sheave sector. A second pawl is also pivoted on the first lever and fixes the relative position of the lever with respect to the sheave sector. When the lever is returned to its rest position, that is, in order to release the brake, the second pawl is lifted by a stop. A heavy spiral spring (equivalent to spring 15 in the Gale patent) then rotates the sheave with respect to the lever, taking all slack out of the cable. When the brake is next actuated, the second pawl engages the sheave sector, locking the lever to the sheave. Not only does this mechanism allow all the slack to be removed from the system, preventing release of the brake in certain circumstances (that is, depending on the point at which the second pawl engages the sheave sector), but the heavy spring mentioned makes the system difficult to operate.
U.S. Pat. No. 4,431,101 to Limbacher shows an automatic adjusting mechanism for a clutch in which a clutch pedal is mounted for rotation concentrically with a second lever. The second lever directly actuates the clutch. The relative angular positions of the pedal and of the second lever about their axis are controlled by a pawl carried by the pedal and acting on a ratchet connected to the second lever. When the pedal travels more than a specified distance during actuation, the pawl is raised by a control member fixed to the vehicle, allowing the pedal to be moved with respect to the second lever. In this way their relative angular position is changed, and the pedal travel required subsequently to actuate the clutch is reduced.
The Limbacher patent thus shows adjusting the mechanism in response to travel of the pedal beyond a specified amount. This system would not appear to allow all the slack to be removed from the mechanism, in distinction to the system, shown in, e.g., the Krautwurst patent discussed above. Of course, the clutch mechanism of Limbacher does not require setting as does an auxiliary brake. Hence the problem of eliminating all slack and thereby preventing release of the brake does not arise. Moreover, the Limbacher mechanism is relatively complex and bulky, is not amenable to fitting into the limited physical space provided in modern vehicles for auxiliary brake actuation mechanisms, would require careful alignment of the control member fixed to the vehicle with respect to the remainder of the mechanism, thus requiring complex assembly steps, and would be relatively costly.